CENTRE FOR OFFSHORE FOUNDATION SYSTEMS …Randolph, Cassidy, Gourvenec & Erbrich 1 ... (onshore)...

16th ICSMGE Osaka, September 2005Challenges of Offshore Geotechnical Engineering

Randolph, Cassidy, Gourvenec & Erbrich

1

Mark Randolph, Mark Cassidy, Susan GourvenecCentre for Offshore Foundation Systems

The University of Western Australia

CENTRE FOR OFFSHORE FOUNDATION SYSTEMS

Challenges of Offshore Geotechnical Engineering

&Carl Erbrich

Advanced Geomechanics, Perth

16th Int. Conf. on Soil Mechanics and Geotechnical EngineeringOsaka, September 2005

16th ICSMGE, Osaka - September 2005 2

Outline

• Background• Trends in offshore design practice

• Site Investigation Practice• Strength measurement in soft sediments• Full-flow penetrometers

• Pile Foundations• Design approaches in sand• Cyclic loading

• Shallow Foundations• Embedded skirts; suction installation • Yield envelopes for combined V-M-H loading

• Mobile Drilling Units• Penetration issues• Force-resultant models for foundation response

• Anchoring Systems• Suction caissons• Static and dynamic embedment of anchors



2


Evolution of Platforms

Piles,gravity bases

or suction caissons Piles

Piles or suction caissons

300 m50 m 500 m 1200 m

Courtesy: Minerals Management Services


Evolution of Floating and Subsea Facilities

Courtesy: Minerals Management Services

Suction caissons

SkirtedFoundations

1750 m 2000 m

Piles Drag anchors



3


Trends in Offshore Design Practice• General

• Enormous costs of site investigation (> $1 million/borehole)• Focus more on capacity than deformations or stiffness• Strength determination: issues of anisotropy, degradation due to cyclic

loading, sampling challenge in deep water soft clays• Environmental loading more significant than for onshore –yield

envelopes replacing bearing capacity factors• Shallow water

• Driven pipe piles – extension of onshore practice, but high capacity• Carbonate sediments – alternative foundations to driven piles• Skirted ‘gravity base’ foundations instead of (onshore) deep basements• Innovative use of short suction caissons in dense sands

• Deep water• TLPs: Special attention to cyclic loading on tension piles• Anchoring technology (drag anchors, suction caissons etc)• Geohazards and pipelines


Geohazards, Pipelines (not covered here)

Courtesy: Norwegian Geotechnical Institute

200 m

Scarps

Canyons

800 to 1000 m

Steep slopes

Example continental slope



4


Offshore Site Investigation

• Conventional drilling (> 30 to 50 m depth of soil)• Piston samples (typically ~80 mm) • Downhole vane and penetrometer tools (inside 150 mm pipe)

• Seabed frames (< 30 to 50 m depth - soft seabed)• Field vane and wheel-drive penetrometer probes

• Large diameter coring (up to 20 to 30 m long)• Fixded piston jumbo core or STACOR (100 to 150 mm diameter)

• Remote robotics – PROD• Small diameter (44 mm) piston sampling to 125 m• Penetrometer testing to 100 m (2 m strokes)

• Full-flow penetrometers• Extensive use of cylindrical (T-bar) and spherical (ball)

penetrometers for soft sediments


Seabed Wheeldrive Penetrometer

To control anddata acquisitionTo control and

data acquisition



5


Portable Remotely Operated Drill (PROD)

Schematic ofdeployed PROD

LaunchingPROD off boat


Comparison of Downhole and Seabed Cone Profile

0

5

10

15

20

25

30

0 200 400 600 800 1000Net cone resistance, qcnet (kPa)

Depth(m)

Downhole

Seabed frame

(Variablydisturbed)



6


Full-Flow Penetrometers

Load-cell

Load-cell

‘Precise’ plasticity solutionsavailable for Nt and Nb

CPT

su = (qt - σv)/Nkt

T-Bar

Section A-AA

A

su = q/Nt

Ball

su = q/Nb

Measured qc first corrected to qt


Field Strength Profiles: Burswood, W. Australia

•Resistance profiles similar• qcone > qTbar

• qball slightly < qTbar

•Comparing shear strengths and penetration resistances:

• N ~ 10 to 12 for T-bar, ball and plate

113 mm

Cone

Plate

T-Bar(250 mm x 40 mm)

Ball

0

2

4

6

8

10

12

14

16

18

200 10 20 30 40 50 60 70 80

Undrained shear strength, su (kPa)

Dep

th (

m)

0 200 400 600 800Net penetration resistance (kPa)

Vane: peakVane: remouldedCAU TXCCAU TXECAU SSUU CompAvg ConeAvg T-barAvg BallPlate

Chung & Randolph (2004)



7

16th ICSMGE, Osaka - September 2005

Resistances Factors For Simple Shear Strength

0

5

10

15

20

25

30

0 5 10 15 20Resistance factors, Nkt-SS and NTbar-SS

Depth(m)

OffshoreWest Africa

OnshoreNorway

OffshoreAustralia

OnshoreAustralia

Open symbols: coneSolid symbols: T-bar

Coefficient of variation of average N values for each site:T-bar: 2 % ; Cone: 14 %


Comments on Offshore Data

• Vane • Typically more scattered than other data (compensating effects

of disturbance, consolidation and rate effects)

• Cone and T-bar penetrometer data• Significant correction of cone data from measured to net values

• Net cone and T-bar resistances similar but cone resistance increases faster with depth than T-bar

• Correlations with laboratory simple shear data suggest resistance factors of ~12, but greater variance among different sites with cone (e.g. Gulf of Mexico: Nkt ~ 17)

• Field tests: typical strain rates ~105 to 107 higher than for standard laboratory test



8


Rate Effects During Undrained Penetration

0.50.60.70.80.9

11.11.21.31.41.5

0.1 1 10 100Rate (mm/s)

Parameter ratio

p/p20 mm/s

Cone: qcnetCone: Du2T-barBall

+ 10%

- 10%

Δu2

qcnet

Hyperbolic sine Logarithmic fit

Rate effect:~13 % per log cycle


Partial consolidation Undrained

Analysis of Variable Rate T-bar Test

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1 10 100 1000 10000V = vd / cv

qTbar ratio: variable/standard

cv = 0.15 mm2/scv = 4.7 m2/yr

Randolph &Hope (2004):

cone resistance

Randolph &Hope (2004):

T-bar resistance

Rate parameter: λ = 0.13

Backbonecurves cv deduced by

matching to backbone curve



9


Rate Effects and Softening

• Shear strength dependency on strain rate• strength increase per log cycle: rate (μ) ~ 5 to 20 %

• Expect ~50 to 100 % increase due to rate effects

• Strain-softening or damage• soil gradually remoulded as it passes through T-bar or ball

mechanism

• net (average) strength therefore less than peak strength

Rate and strain-softening effects compensate;quantify using strain path approach

⎟⎟⎠

⎞⎜⎜⎝

⎛γγ

μ+=ouo

u log1ss

&

&

( ) ( ) 95/3remremuus e1s/s ξξ−δ−+δ==ξδ


T-bar Factors: Strain Rate and Softening

5

10

15

20

25

0 0.05 0.1 0.15 0.2 0.25Rate parameter μ

NTbar ξ95 = 50ξ95 = 25ξ95 = 15

No strainsoftening

Parametersα = δ rem = 0.2ξTbar = 3.85

ξ95 = 10

For typical rate parameter, μ ~ 0.1, NTbar ~ 11 to 13

Corresponding theoretical range for Nball ~ 13 to 16

Strain to 95 % remoulded



10


Remoulded Strength and Sensitivity

• Penetrometers ‘measure’ rate-enhanced but partially softened average strength

• Sensitivity gauged by comparing extraction and penetration resistances • Monitor cone resistance during extraction

• Fully remoulded shear strength determined by cycles of penetration and extraction• Full-flow penetrometers (T-bar, ball) superior to cone

for this


13.613.8

1414.214.414.614.8

-400 -200 0 200 400 600Penetration resistance, q (kPa)

Depth(m)

Cyclic T-bar Results: Soft Clay Onshore Australia

3.63.8

44.24.44.64.8

-200 -100 0 100 200 300Penetration resistance, q (kPa)

Depth(m)

Fully remoulded resistance

~ 20 % of initial value (St ~ 5)



11


Summary of Overall Rate Effects

• Rate effects significant• Field data show rate dependency from 10 % to 20 % per log cycle

• High plasticity clays show particularly high rate effects

• Partially drained response for V = vd/cv < ~ 30• Burswood: d = 40 mm, cv ~ 0.15 mm2/s: vmin = 0.1 mm/s

• Partial drainage detectable at V ~ 100 from Bq

• Benefits from testing at varying rates:• Assessment of rate effects

• Determination of degree of consolidation and cv


Example Application: Spudcan in Calcareous Silt

0

5

10

15

20

25

0 0.5 1 1.5 2

Penetration Resistance (MPa)

Depth(m)

T-barCone(net)

Spudcan

Bearing pressure: 0.35 MPa

Spudcan

T-bar Spudcan

Strain rate (v d) 0.5 0.5 x 10-3 s-1

Consolidation (vd) 10-3 10-1 m2s-1

Erbrich (2005)

cone



12


Pile Foundations

• Pile types and construction• Steel pipe piles near universal except in carbonate soils

• Main construction risk is refusal (collapse at tip – Goodwyn, Valhall)

• Carbonate soils: drilled and grouted piles generally preferred; driven closed-ended pipe piles a possible alternative

• Hole collapse a major risk for drilled and grouted construction, but alternative of grouted driven piles unproven in field applications

• Design issues• Friction ratio, α, in clay decreasing function of su/σ′vo (or OCR)

• Shaft friction in sand degrades during further pile installation

• End-bearing capacity in sand a key issue

• Explicit modelling of cyclic loading where strain-softening might be catastrophic (tension piles, piles in carbonate soils)


64 m

Traditional Steel Jacket Structure With Driven Piles

Pile diameters: 1.5 to 2.5 mHammer energies: up to 3000 kJ (180 tonne ram)

maximum water depth now ~2,000 m



13


Pile Capacity in Sand – Recent Reassessment

• Modern approach entirely based on cone resistance, qc

• End-bearing (Xu et al., 2005)

• Design for capacity mobilised at displacement of 0.1D

• Fractional mobilisation of qc depends on displacement ratio of pile

• Closed ended (or plugged) : qbu ~ 0.6qc – independent of diameter

• Open ended: qbu ~ qc[0.15 + 0.45 Ar] (typically ~ 0.2qc minimum)

• Shaft friction (White, Lehane, Schneider, 2005)

• Maximum value close to pile tip proportional to qc x f(Area ratio: Ar)

• Shaft friction degrades due to further pile installation: f(h/D)

• Friction degradation limits use of driven piles in carbonate sands

Design approach:Major extroplation in pile size required


End-bearing Capacity in Sand

Closed endedqbu = 0.6qc

Area ratio of pile base0 0.2 0.4 0.6 0.8 1

qb-0.1/qc

0

0.2

0.4

0.6

Typicaloffshore

qbu = 0.2qc

Open ended database

Lehane, Schneider& Tao (2005)

0 0.2 0.4 0.6 0.8 1Pile diameter (m)

qb-0.1/qc

0

0.2

0.4

0.6

0.8

1

Closed ended database



14


Shaft Friction Degradation

cv

3.0rc

f tanD/h

Aq03.0δ=τ

Cone resistanceqc/100

h/d = 25

h/d = 14h/d = 4

Data from Lehane et al (1993)

Cyclic shearing undernormal stiffness condition

(White & Bolton 2004)

Lehane, Schneider& Tao (2005):


Grouted Pile Construction and Design

0

0.05

0.1

0.15

0.2

0.25

0.3

0 50 100 150 200 250qc/pa

p /qc

50 kPa75 kPa100 kPa150 kPa200 kPa250 kPa300 kPa500 kPaTrend upper boundTrend lower bound

Grouting pressures

drivinghammer

primarypile

Installationof primary pile

Drilling borehole

Grouting insert pile

rotary drill

insert pile

grouting string

Drilled & grouted pile design: τp = 0.02qc

c

p

qτ

ac p/q

Laboratory test results for grouted driven piles



15


Calibration of Load Transfer Models for Cyclic Loading

• Constant Normal Stiffness (CNS) tests on core samples, setting normal stiffness to k = 4Gmax/d, where d is the pile diameter

• Calibration using load transfer model in RATZ (Randolph, 2003)

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

-6 -4 -2 0 2 4 6 8 10 12Horizontal displacement (mm)

Normalised shear

stress, τ/τp

CNS Test

RATZ calibration

rapid softening

low cyclicresidual friction

partialrecovery


Example Application for Drilled & Grouted Pile

• Ideal shaft capacity of 82 MN is reduced to 58 MN (71 % of ideal) due to progressive failure with strain-softening load transfer

• Cyclic loading from 0 to 39 MN (67 % of monotonic capacity) reduces capacity further, to 51 MN (63 % of ideal shaft capacity)

-100-50

050

100150200250300350

0 5 10 15 20 25 30

Local pile displacement (mm)

Mobilised shaft

friction (kPa)

Monotonic

Cyclicresponse

Element response at 20 m depth

2 m

50 m

40 mmwall

80 mmwall

60 mmwall



16


Effect of Cyclic Loading on Pile Capacity

-10

0

10

20

30

40

50

60

0 20 40 60 80Pile head displacement (mm)

Pile

load

(MN

) Pile headDepth: 20 mDepth: 30 m

Depth: 40 m

Monotonic

0

10

20

30

40

50

60

0 100 200 300 400 500Shaft friction (kPa)

Dep

th (m

)

Initial peak

Initial residual

Post-cyclic

Pile load-displacement

response

Profiles of shaft friction before and after storm


Summary Comments on Pile Foundations

• New design approaches based on limited database• High quality tests in sand: 42 closed ended, 32 open ended

• Average diameter (open ended): 0.73 m; average length 25 m

• Typical offshore pile: diameter > 2 m; length 50 to 80 m

• Significant extrapolation required

• Cyclic loading effects dominate pile design• Driven piles: friction degradation during installation linked

directly to number of "cycles" (or hammer blows)

• Grouted piles: significant loss of shaft friction due to operational cyclic loading



17


Shallow Foundations

• Gravity-base foundations • Large (> 400 m tall, plan area > 15,000 m2)• High environmental loading (cyclic, large moment component)

• Embedded skirts• Serves equivalent purpose as deep excavation, encasing soft sediments• Consolidation within skirts contributes to settlement (long time scale) • Suction-assisted installation and local strengthening of soils• In sands, suction installation cause quasi-piping failure within soil plug

• Yield envelopes for combined V-M-H loading• Design codes gradually replacing factors for inclined and eccentric

loading by yield envelopes in V-M-H space• Uniaxial bearing capacities (Vult, Mult, Hult) need to allow for strength

gradient and embedment


Troll – Concrete Gravity Base Structure (GBS)

Troll (1995)Total height ~ 450 m; foundation width 150 m, skirt depth 36 m



18


Wandoo Gravity Base – NW Shelf, Australia

1 m of dense carbonate sandoverlying calcareniteCyclic sliding critical

(Humpheson, 1998) Shear stress ratio τ/σ′v

Simple shear (with pre-shear) Simple shear

(no pre-shear)

Triaxial CAU

Triaxial CIU100

10

0.1

1

0.010.1 0.2 0.3 0.4 0.5

v

N/uσ′

Δ=β

4.2/11 v10 −σ′τ=β


Bayu Undan Gravity Jacket – Timor Sea, Australia

Muddy silt(excavated by jetting)

above layered calcarenitebearing capacity critical

(Neubecker & Erbrich, 2004)

-2000

-1500

-1000

-500

0-0.5-0.4-0.3-0.2-0.10

Displacement (m)

Bea

ring

pres

sure

(kPa

)

Intact caprock:Capped Drucker Prager

Cracked caprock: capped Drucker Prager

Initially intactcaprock: Cam Clay

Displacement (m)



19


Yolla Hybrid Gravity Base – Bass Strait, Australia

Uncementedcarbonate

sands & siltsmomentloadingcritical

(Watson & Humpheson, 2005)

50 m square, 5.4 m deep skirts(suction-assisted installation)


Bearing Capacity of Shallow Foundations in Clay

VH

M = f(Hzw)

zw

z

h

suo su

k

sum

Shear strength

κ = kD/suoD

Nc = qu/suo

Bearing capacity expressed as

With Nc a function of:strength profile, κ = kD/suo

loading (H/V, M/DV) shape, embedment



20


Shallow Foundations on Clay – Vertical Loading

B or D

0

2

4

6

8

10

12

0 0.5 1 1.5 2Embedment, h/D

Bearingcapacity

factorNc

kD/sum = 10

kD/sum = 01

25

Lower bound results (circle)Martin (2001)

z

h

suo su

k

sum

Nc = qu/suo

Shear strength

κ = kD/suo


Yield Envelopes for Combined V-M-H Loading

• Establish Vult, Mult, Hult for uniaxial loading• Develop generic 3-dimensional shape, allowing for

• Whether suction (tension) allowed at foundation-soil interface

• Strength profile (kD/suo)

• Shape of foundation

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

M/ADsuo

H/Asuo

ISO(no tension)

FEA(allowing tension)

Yield envelope in M-H planeCircular foundationHomogeneous soilV = 0.5Vult

neglectedcapacity



21


Three-Dimensional Yield Envelopes

M/ADsuo

0.2

0.6

0.4

0.8

H/Asuo

V/Asuo

0

1

2

3

4

5

6-1.0

-0.5

0.0

0.51.0

V/Asuo

H/Asuo

M/ADsuo

0.0

0.2

0.6

0.4

0

1

2

3

4

5

6-1.0

-0.50.0

0.51.0

based on strip footinginclination andshape factors

FE Analysis(circular, allowing tension)

ISO Guidelines (no tension)


Enhanced Capacity due to Embedment

Vertical ddh

Horizontal

Moment

Deeper mechanisms involve stronger soil- and also greater tension capacity



22


Mobile Drilling Rig Foundations

• Self-elevating mobile units (jack-ups)• Buoyant triangular hull, 3 independent legs

• Spudcan foundations up to 20 m diameter

• Safety margin provided by preloading to typically twice operational load

• Highest prevalence of accidents in the offshore field

15 - 20 m

~130 m

Arabdrill19 Sep 2002


preloadingd / D = 1.0

1.26 D

0.5 D

Installation Procedure for Jack-up Rigs

transport or floating operating



23


Spudcan Penetration

• Circular, shallow conical, foundations for mobile drilling (jack-up) units• Typically 15 to 20 m diameter, penetrating 1 to 3 diameters

• Cavity depth above spudcan critical for• Bearing capacity

• Extent to which suctions may develop beneath spudcan during uplift or moment loading

H

V

p

D

Cavity

K-latticeleg

Spudcan

Hosseinet al (2005)


0.01

0.1

1

10

0.001 0.01 0.1 1 10su / γ′D

H / DCentrifuge test (average)Centrifuge test (bound)

γ′ = 17 kN/m3, D = 18 mγ′ = 7 kN/m3, D = 18 mγ′ = 17 kN/m3, D = 12 mγ′ = 7 kN/m3, D = 12 mγ′ = 17 kN/m3, D = 6 mγ′ = 7 kN/m3, D = 6 m

Unstable

Stable

Designrange

FE Analyses

Stable Cavity Depth During Spudcan Penetration

55.0uD

sDH

⎟⎟⎠

⎞⎜⎜⎝

⎛γ′

≈

Hossein et al (2005)



24


Bearing Capacity Factors for Spudcan

6

7

8

9

10

11

12

0 0.5 1 1.5 2

Penetration, p/D

Nc

Smooth interface

Upper and Lower BoundsSalgado et al. (2003)

Increasing su/γ 'Dfrom 0.14 to 0.36

Initiation of back-flow above spudcan

Hossein et al (2005)

cavity:qu = Ncsu + σ'v

local flow:qu = Ncsu


Expansion of Yield Surface by Penetration

V

M/2R

Yield surface in (V, M/2R, H)load space

V0

Plastic Penetration (wp)

H Expansion of surface

V

M/2R


V

M/2R


V0

Plastic Penetration (wp)

H Expansion of surface

Yield surfacein (V, M/D, H)load space

M/D



25


Consider cases wherespudcans are:

• pinned (no moment)• encastre (no rotation)• resting on "sand"

(partial restraint)

Example Dynamic Analysis of Jack-Up Rig

52m

90m115m

UPWAVE DOWNWAVEDOWNWAVE

WAVES


Horizontal Hull Movement During Wave Loading

pinnedsandencastre

downwave legsupwave leg

wav

e el

evat

ion

(m)

0-4-8

-12

48

12

-0.5

0.50

-1-1.5

11.5

hull

sway

(m

)

time (s)-50 -50 1000



26


Extension of Footing Models for Cyclic Loading

• Force-resultant models a major advance, but• Need to allow for gradual yielding (non-linearity) prior to yield

• Experiments show hysteretic unloading-reloading response

• Real soil exhibits cyclicdegradation

Horizontal Load

Vertical Load

Yield envelope under monotonic preload

Degraded cyclic yield envelope

Material factored cyclic yield envelope

'Equivalent' preload

Safe zone (shaded)

Unsafe zone

Applied preload


Anchoring Systems

• Deep-water developments increasingly dominate• Floating production units anchored to seabed

• Catenary (chain) moorings: quasi-horizontal loading of anchors

• Trend towards taut, or semi-taut, moorings (polyester cable)

Gravity anchors Embedded anchorsBox Anchor pileGrillage and berm Suction caisson

Drag anchor (fixed fluke)Vertically loaded drag anchorSuction embedded plate anchorDynamically embedded anchor



27


Example Anchors

Grillage and berm anchorshallow water gravity anchor

T

Suction caissonsdeep water taut-wire anchor

rock fill


Suction Anchors - Installation

• Self-weight penetration, typically by ~50 % of length

• Pumping to final embedment - avoiding• Buckling of caisson (e.g. Devil's Tower, Gulf of Mexico)

• Upward plug failure

• Key uncertainties:• Bearing resistance of internal stiffeners

• Effects of gradual remoulding of soil

• Flow around internal ring stiffeners andeffect on subsequent internal shaft friction

Vent closed: pumping

γ'

αisuαosu

q = Ncsu + γ'L

L

W

Δu

D(Area, A)

t



28


Measured and Predicted Under-Pressure

0

5

10

15

20

25

30

35

0 50 100 150

Required Underpressure (kPa)D

epth

(m)

M easured

P1

P2 Total

P2 Effective

P3

P4

Consolidationduring delay

δ = 12º δ = 17º

0

2

4

6

8

10

12

14

16

18

0 25 50 75 100Required Underpressure (kPa)

Dep

th (m

)

Diana (Gulf of Mexico) Girassol (West Africa)Andersen et al (2005)


Lateral Capacity of Suction Anchors

Caisson

D*

LD2D1

Taut wire

Catenary mooring

d

θ2

θ1

h

Centre ofrotation

O

Horizontal caissonvelocity at depth, z

⎟⎟⎠

⎞⎜⎜⎝

⎛

−

−= *oh

Dhzhvv

ψ⎟⎟⎠

⎞⎜⎜⎝

⎛

−

−⎟⎠

⎞⎜⎝

⎛=

μ

cosDhzh

rr

vv *o

or

Horizontal soil velocity

D* ~ 0.7L for optimum lateral capacity

(Murff & Hamilton, (1993)



29


Soil Failure Mechanisms for Suction Caissons

0

2

4

6

8

10

12

0 1 2 3 4 5Aspect ratio, L/d

Normalised capacityH/Ld(su)avg

No rotation

Free rotation(loaded at seabed)

Uniform strength with depth

Strength proportional to depth

Strength proportional to depth

( )averageuu sLd10H ≈


Interaction Diagrams Under Combined Loading

0

5000

10000

15000

0 5000 10000 15000 20000Horizontal load (kN)

Ver

tical

load

(kN

)

FE: no crackFE: crack allowedAGSP ANC: 2-SidedAGSP ANC: 1-Sided

0

10000

20000

30000

0 10000 20000 30000 40000Horizontal load (kN)

Ver

tical

load

(kN

)

FE: no crackFE: crack allowedAGSP ANC: 2-SidedAGSP ANC: 1-Sided

3dL5.4b;

dL5.0a with 1

VV

HH

b

u

a

u+=+==⎟⎟

⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛

L/d = 3 L/d = 5

Caissons: 5 m diameter; su = 10 + 1.5z kPa, γ' = 7.2 kN/m3

Failure envelope:



30


Drag Embedment Anchors

• Traditional anchor type, with holding capacity to weight ratios of 20 to 30• Unsuitable to withstand significant vertical load

• "VLA" (vertically loaded anchor) for deep water• VLA comprises ~10 m2 plate anchor with lightweight shank

• VLA mechanism aligns anchor force normal to plate

Bruce DENNLA anchor(inverted, lying on shank)

Shank

Fluke


Kinematic Analysis of Anchor Embedment • Treat fluke as plate with defined yield envelope in

normal, parallel and moment load space(Bransby & O'Neill, 1999)

• Allow for additional soil resistance on shank

• Model chain using simple relationships

θa

Ta

To

D

Seabed

θo

Chain-soil interaction

Chain:effective width, b

friction coefficient, μ

Q = NcbsuμQ

( ) QDQdz2

T D

0

2o

2a

a =≈θ−θ ∫

( )oaeTT

a

o θ−θμ≈



31


Plate Anchor Interaction Envelopes and Response

0

1

2

3

4

5

0 2 4 6 8 10 12Normal factor, Nn

L/t = 7, fully rough

Normal-parallel

Normal-moment

-1

-0.5

0

0.5

1

00.20.40.60.81fn/fnmax

Yield locus:fs = 0

-1

-0.5

0

0.5

1

0 0.2 0.4 0.6 0.8 1

fs/fsmax

m/m

max

Yield locus:fn = 0

Nm : Nn curve

Ns : Nn curve

Strip anchor(Murff et al, 2005)Kinematic analysis

(O'Neill et al, 2003)

u

normaln Ls

fN =

u

shears Ls

fN =

u2m sLmN =


Anchor: Overall Performance

Kinematic analysis (O'Neill et al, 2003)

0

1

2

3

0 10 20 30 40

Drag length x/L

Pade

ye d

epth

D/L

0

5

10

15

20

0 10 20 30 40

Drag length x/L

Anc

hor e

ffici

ency

-0.2

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40Drag length x/L

Nor

mal

ised

fluk

e lo

ad .

fn/fnmax

fs/fsmax

m/mmax



32


•Anchor installed by free-fall from ~50 m above seabed (20 – 30 m/s)

•Loading mainly resisted by friction along soil-shaft

DPA - 1:3 model scale tests inTrondheim Fjord, Norway 2003

Torpedo Anchor: Brazil 2003

Dynamically Embedding Anchors

DPA Torpedo Anchors

10-15 m


Centrifuge Model Tests

Anchor “A” (4 flukes) Anchor “B” (3 flukes) Anchor “C” (0 flukes)

Water

650 mm

300

mm

1

2

Installation guide

Model DPA

Load cellActuator

Kaolin

Strongbox

Beam centrifuge – 200 kg at 1.8 m radius at 200 g.



33


Dynamic Anchor Capacity

O’Loughlin et al. (2004)

300

250

200

150

100

50

0

0 2 4 6 860

50

40

30

20

10

00 50 100 150 200

PETROBRASfield trials

Prototype Vertical Capacity, Fv (MN)

4 fluke DPA 3 fluke DPA 0 fluke DPA Predicted

0 fluke DPA

3 fluke DPA

4 fluke DPA

Prot

otyp

e em

bedm

ent (

m)

Model Vertical Capacity, Fv (N)

Mod

el E

mbe

dmen

t (m

m)

Centrifuge models:


Deep-Water Anchoring Options

Anchor TypesSuction Caissons

Drag EmbeddedPlate Anchors

(VLAs)

Suction EmbeddedPlate Anchors

(SEPLAs)

Dynamically Embedded Anchors

Advantages- proven installation record- wide experience- robust design

- low weight- wide experience- small, high efficiency

- proven suction technology- robust design- lowest cost of anchor

- conventional (API) design- economical to fabricate- simplest to install

Disadvantages- need ROV to install- concern for layered soils- awkward to transport

- uncertain installation- need ROV & 2 vessels- poor quality assurance

- patented design- need keying & proving- long installation times

- patented designs- low efficiency- limited field experience

Ehlers et al. (2004)



34


Concluding Remarks

• Exploiting hydrocarbon resources in deep-water an increasing challenge, especially site characterisation of very soft sediments

• Calibration of in situ test results by variably disturbed laboratory strengths is a losing strategy

• Strive for improved design basis with foundation and anchor performance related directly to in situ penetration resistance

• Cost-benefits for improvement in design approachesfor conventional shallow and pile foundations

• Simple and robust anchoring systems a priority


Acknowledgements

Grateful acknowledgements to:

Australian Research Council’s Research Centres Program for funding COFS (Centre for Offshore Foundation Systems)

Many colleagues at COFS and Advanced Geomechanics for their input into ideas presented here

Colleagues throughout the world, in particular at the Norwegian Geotechnical Institute and the

Offshore Technology Research Center

CENTRE FOR OFFSHORE FOUNDATION SYSTEMS …Randolph, Cassidy, Gourvenec & Erbrich 1 ... (onshore)...

Documents

Transcript of CENTRE FOR OFFSHORE FOUNDATION SYSTEMS …Randolph, Cassidy, Gourvenec & Erbrich 1 ... (onshore)...